Igniter plug



R. D. WILLIS Jan. 11, 1966 IGNITER PLUG Filed May :2. 1960 United States Patent 3,229,032 IGNITER PLUG Raymond D. Willis, Flint, Mich, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 2, 1960, Ser. No. 25,979 3 Claims. (Cl. 174-152) This invention relates to igniter plugs for gas turbine engines and the like and to a method for making same. More specifically, the invention relates to an improved method and means for securing and sealing the center electrode into the ceramic insulator of such an igniter.

In the design and construction of spark plugs or igniter plugs it is highly desirable if not imperative to include structure which will prevent the electrode from dropping free of the plug in the event there should be a failure of the seal between the electrode and the insulator. In most spark plug constructions this is accomplished by forming the top of the center electrode with anintegral head portion which rests against an internal shoulder in the insulator centerbore so as to preclude any downward movement of the electrode with respect to the insulator. However, in igniter plug designs wherein the firing tip of the center electrode comprises an enlarged button which abuts the lower end of the insulator, it is not feasible to use an integral head portion at the top of the electrode for the reason that it would then be extremely difficult if not impossible to form the assembly of the electrode with the insulator during plug manufacture.

The present invention has as an object the provision of an igniter plug of the type having an electrode with an enlarged firing tip exterior of the insulator and wherein the electrode-to-insulator joint provides optimum assurance against loosening of the center electrode from the assembly even under severe abnormal operating conditions. The invention has as another of its objects the provisionof an improved method for forming a center electrode-to-insulator joint and seal. Other objects and advantages of the invention will appear more clearly from the following detailed description of a preferred embodiment thereof made with reference to the accompanying drawings in which:

, FIGURE 1 is a side view in'section of an igniter plug constructed in accordance with the invention;

FIGURE 2 is an enlarged view of a portion of the j igniter plug shown in FIGURE 1 but during an early step in its manufacture; 7

FIGURE 3 is a View similar to FIGURE 1 but illustrative of a subsequent step in manufacturing; and

FIGURE 4 is a perspective view of the gasket element of the assembly shown in FIGURE 2.

Referring now to FIGURE 1, the igniter plug comprises a metal shell 2, a ceramic insulator 4 secured within the shell and having a centerbore extending therethrough, and a center electrode 6 within the insulator centerbore. The lower end of the center electrode, which is of a high heat resistant metal, preferably Inconel, is formed with an enlarged button portion 8 which abuts the bottom of the insulator and forms an annular spark gap with the lower end of the metal shell 2, the shell itself serving as the ground electrode. The upper end of the center electrode serves as a contact for electrical connection with the ignition system. The present invention relates to structure and method for securing the center electrode within the insulator centerbore as will now be described, it being understood that the precise shape of the shell, the exterior shape of the insulator and the means for securement of the insulator within the shell form no part of the invention and may be selected as desired.

The insulator centerbore has a lower portion 10 which is of about the same diameter as the center electrode and an upper portion 12 of enlarged diameter, the junction between the upper and lower portions constituting an inwardly extending annular shoulder 14. The center electrode is formed with an elongated portion of reduced diameter, or an annular groove, 16, which is positioned at the shoulder 14 as shown, the junction between the portion 16 and the upper end of the center electrode defining an outwardly extending annular shoulder 18. A gasket 20, preferably of sheet nickel, is wedged tightly between the center electrode portion 16 and the ceramic insulator so as to form a frictional contact seal.

To hermetically seal the centerbore, an annular mass of glass 2 above the gasket 20 is bonded to the electrode and to the insulator. In the preferred embodiment, the upper end of the electrode is made of Kovar (about 54% Fe, 28% Ni, 18% Co) which is welded to the Inconel (about Ni, 14% Cr, 6% Fe) lower end of the electrode, as shown at 24. The composition of the glass is, of course, selected with a view to matching its coefficient of expansion to that of the Kovar. The particular glass used in the embodiment shown is sodium boroaluminum silicate glass, the following being a typical analysis: 59% SiO 7.5% A1 0 7.5% Na O, 25% B 0 plus minor amounts of alkaline earth oxides, Fe O and TiO Under normal and even severe operating conditions, the glass seal 22 and the frictional contact between the wedged gasket 20 and the electrode prevent any loosening of the electrode within the insulator centerbore. Further, even if by way of abnormal, extremely severe conditions the glass seal should become so heated as to melt and even if the frictional contact between the wedged gasket and the electrode should loosen, the electrode will nevertheless be prevented from dropping free of the assembly since the center opening in the gasket is considerably smaller than the external diameter of the electrode shoulder 18. In this connection, it should be noted that the electrode is of one-piece construction from its firing tip to above the location of the shoulder 18. Hence there can be no loss of the electrode from the assembly due to failure in a bonded or mechanical joint in the electrode. No disadvantage arises by way of a small portion of the electrode in bonded contact with the glass 22 being of metal, i.e., Inconel, which has a coefhcient of expansion not closely matched to that of the glass.

The method for manufacturing the insulator-center electrode assembly is as follows: The electrode is fabricated by welding the Kovar portion to the Inconel portion and by machining to final shape as shown, the reduced diameter portion 16 being spaced from the button 8 a distance equal to that from the bottom of the insulator to the shoulder 14. It is then inserted into the insulator centerbore from the bottom until the button 8 is snug against the bottom of the insulator. A conical shaped nickel gasket 20 is inserted in the top of the insulator around the center electrode to a position of rest against the insulator shoulder 14 and with the apex end of the conical shaped gasket pointing upwardly. This is shown in FIGURE 2; it will be noted that the gasket in its conical shape has an opening of a size to permit its passage over the top portion of the centerwire and that the length of the reduced diameter electrode portion 16 is at least as great as the axial length of the conical shaped gasket. With the gasket so positioned, a tubular tool 26 is inserted into the centerbore around the center electrode and, by means of this tool, pressure is applied to the gasket adjacent and around the center opening therein so as to force it into its fiat shape as shown in FIGURE 3. The gasket is sized such that when it is pressed into its planar shape it is wedged tightly between the insulator and the reduced portion 16 of the electrode. During the pressing operation the entire assembly is, of course, positioned in a suitable fixture so as to prevent any downward movement of the electrode with respect to the insulator during pressing.

After the gasket is pressed into its planar shape and the tool 26 removed, a quantity of powdered glass of the desired composition is placed in the centerbore over the gasket and the assembly is then placed in a furnace to cause the powdered glass to melt and thereby bond to the electrode and the insulator as Well as to the gasket. After removal from the furnace and subsequent cooling the insulator-center electrode assembly is complete and may be assembled with the metal shell and other igniter plug components in a conventional manner.

The method and structure of this invention provides an excellent igniter plug insulator-to-electrode sealed joint with optimum assurance against loosening of the electrode Within the insulator, and at very low manufacturing cost. Because the electrode is of unitary construction from its position of interlock with the insulator to the firing tip, the possibility of electrode disassembly from the igniter plug due to a joint failure is precluded. This is an especially important feature since an igniter plug for an igniter plug for an aircraft jet engine should ideally be able to withstand the most severe abnormal conditions that might arise due to engine malfunction Without giving rise to the possibility of plug disassembly which would compound the engine malfunction.

It will be understood that while the invention has been described specifically with reference to a preferred embodiment thereof, changes and modifications may be made all within the full and intended scope of the claims which follow.

I claim:

1. An igniter plug comprising a ceramic insulator having a centerbore therethrough, said centerbore having a lower portion of relatively small diameter and an upper portion of relatively large diameter with the junction between said portion constituting an inwardly extending annular shoulder in said centerbore, an elongated electrode extending through said centerbore having an enlarged portion exterior of and abutting the bottom of said insulator and a portion of reduced diameter intermediate the ends thereof and adjacent the shoulder in said centerbore, and a flat annular metal gasket disposed in the enlarged portion of said insulator centerbore wedged between said insulator and the reduced diameter portion of said center electrode and positioned perpendicularly to the axis of said center electrode, with the outer and inner rims of said flat annular metal gasket respectively tightly engaging said insulator and the reduced diameter portion of said center electrode to thereby permanently lock said electrode in said insulator centerbore.

2. An igniter plug comprising a ceramic insulator having a centerbore therethrough, said centerbore having a lower portion of relatively small diameter and an upper portion of relatively large diameter with the junction between said portions constituting an inwardly extending annular shoulder in said centerbore, an elongated electrode extending through said centerbore having an enlarged portion exterior of and abutting the bottom of said insulator and a portion of reduced diameter intermediate the ends thereof and adjacent the shoulder in said centerbore, and a fiat annular metal gasket in the enlarged portion of said insulator centerbore wedged between said insulator and the reduced diameter portion of said center electrode and positioned perpendicularly to the axis of said center electrode, with the outer and inner rims of said flat annular metal gasket, respectively tightly engaging said insulator and the reduced diameter portion of said center electrode to thereby permanently lock said electrode in said insulator centerbore, and an annular mass of glass in the enlarged portion of said centerbore bonded to the insulator and to the center electrode to form a hermetic seal therebetween.

3, An igniter plug as set forth in claim 2 wherein an upper portion of said electrode above said portion of reduced diameter is of a metal having a coeificient of expansion substantially matching that of said glass and wherein the entire portion of said electrode below said upper portion is of unitary construction and is welded to said upper portion.

References Cited by the Examiner UNITED STATES PATENTS 1,315,298 9/1919 Genn 313-136 X 1,455,852 5/1923 Pyle 313-136 X 1,556,241 10/1925 Mueller 285-340 X 1,695,632 12/1928 Campbell 313-136 2,164,311 7/1939 Doren 313-136 2,248,415 7/1941 Schwartzwalder 313-136 X 2,296,045 9/1942 McDougal 313-136 X 2,356,104 8/1944 Tognola 313-136 2,436,644 2/1948 Halstead 313-136 X 2,757,945 8/1956 Bingham 285-340 2,806,971 9/1957 Twells et a1 313-136 2,867,683 1/1959 Smith 174-152 2,891,187 6/1959 Bretsch et al 313-136 2,969,582 l/1961 Meredith et al 29-25.12 2,998,632 9/1961 Yenni et al. 2925.12 3,055,086 9/1962 Hoganson 29-522 X JOHN W. HUCKERT, Primary Examiner.

ARTHUR GAUSS, JAMES D. KALLAM, DAVID J.

GALVIN, Examiners. 

1. AN IGNITER PLUG COMPRISING A CERAMIC INSULATOR HAVING A CENTERBORE THERETHROUGH, SAID CENTERBORE HAVING A LOWER PORTION OF RELATIVELY SMALL DIAMETER AND AN UPPER PORTION OF RELATIVELY LARGE DIAMETER WITH THE JUNCTION BETWEEN SAID PORTION CONSTITUTING AN INWARDLY EXTENDING ANNULAR SHOULDER IN SAID CENTERBORE, AN ELONGATED ELECTRODE EXTENDING THROUGH SAID CENTERBORE HAVING AN ENLARGED PORTION EXTERIOR OF AND ABUTTING THE BOTTOM OF SAID INSULATOR AND A PORTION OF REDUCED DIAMETER INTERMEDIATE THE ENDS THEREOF AND ADJACENT THE SHOULDER IN SAID CENTERBORE, AND A FLAT ANNULAR METAL GASKET DISPOSED IN THE ENLARGED PORTION OF SAID INSULATOR CENTERBORE WEDGED BE- 